Peripheral monitoring devices for working machines such as hydraulic excavators monitor the periphery of the working machine by means of a camera and/or the like to prevent the machine from coming into contact with any obstacles (persons and objects) present around the machine during work. Techniques relating to devices of this type are intended to allow an operator to easily and readily recognize the relationship in position between the working machine and the obstacles existing around it. Among these techniques is one that achieves its intended purpose by displaying three images in superimposed form on a display device. One depicts an attitude and working zone of the working machine during the work, one is obtained by converting a camera-acquired image of the working machine periphery into a bird's-eye image of the machine as viewed diagonally from above, and one depicts an obstacle appropriately detected by the camera or other means. JP-2008-248613-A, for example, discloses the technique.